1. Field of the Invention
The present invention relates to a flat panel display device, and more particularly, to a flat panel display device having improving a light coupling efficiency from a self-luminant device without sacrificing image sharpness.
2. Description of the Related Art
In general, a light coupling efficiency of a flat panel display using a self-luminant device is determined by refractive indices of layers from the light emitting layer to the outside of the flat panel display. One portion of the display that lowers the light coupling efficiency is at the interface of a transparent substrate having high refractive index and air having low refractive index. At such an interface, total reflection can occur when the incident angle of light is greater than the critical angle, thus lowering the light coupling efficiency.
In the conventional flat panel display having the self-luminant device, the light coupling efficiency of light emitted from the self-luminant device to air via the substrate is based on an equation (Nout/Nin)2/2. In the above equation, N denotes the refractive index.
When the light coupling efficiency in a state in which a glass substrate is used in the above equation, the refractive index Nin of the glass substrate is about 1.52 and the refractive index of the air Nout is about 1.00, and the light coupling efficiency is about 21.64%. That is, more than 70% of light incident on the substrate is extinguished in the substrate.
In addition, because the light emitted by the self-luminant device is emitted in all directions, there are some solutions for solving the above problem. For example, if a supplied voltage is rises, the brightness can be improved, however, a capacity of a battery must be increased, resulting in an increased weight of the device and reduced life span of the battery and the self-luminant device. Therefore, there are some suggested technologies for reducing the supplied voltage and improving the brightness.
Japanese Laid-open Patent No. hei 4-192290 discloses an electroluminescence (EL) device including a plurality of micro lenses for condensing the light, each of which has a size that is equal to that of a pixel or larger and formed on an exterior surface of a light-transmittable substrate on which the inorganic EL device is formed. The light that is incident onto an interface between the light-transmittable substrate and the air at a critical angle or larger has an incident angle that is less than the critical angle in the micro-lens to reduce the total reflection, and an emitting direction of the light is directed to a predetermined direction to increase the brightness in that direction. However, according to the above invention, since the EL device is a surface source, diffused EL light is generated inevitably when the micro-lens has a size equal to or larger than the size of the pixel. In addition, images are overlapped due to adjacent EL devices, thus the sharpness of the image is degraded.
Japanese Laid-open Patent No. hei 7-037688 discloses an EL device that is formed on a substrate having a high refractive portion that is made of a material having higher refractive index than that of surrounding material in a thickness direction thereof. The light of the EL device transmits the high refractive portion and is emitted, in order to increase the light coupling efficiency. However, in this invention, since the EL light transmitting through the high refractive portion is the diffused light as shown in FIG. 1 of the invention, the brightness is not greatly improved.
Japanese Laid-open Patent No. hei 10-172756 discloses an organic EL device in which one or a plurality of condensing lenses are formed between a lower electrode and an exterior surface of a light-transmittable substrate that forms an organic EL device, and the organic EL device corresponds to the condensing lenses. The light of the EL device transmitting the condensing lens is incident into the interface between the substrate and the air at the critical angle or lower, in order to increase the light coupling efficiency. However, according to that invention, images are overlapped with those of adjacent EL devices, thus degrading the sharpness of the image.